The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos ⊖, where ⊖ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic (AFM) material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
The ever increasing demand for increased data rate and data capacity has lead a relentless push to develop magnetoresistive sensors having improved signal amplitude and reduced track width. However, as sensors become smaller the free layers in such sensors become less stable. Free layer stability refers to the ability of the magnetization of the free layer to remain biased in a desired direction parallel with the ABS. This instability in very small sensors is due to several factors, including the reduced area on which a biasing layer can act magnetostatically to provide a bias field. Another factor affecting the stability is the small size of the free layer itself, which makes the free layer inherently magnetically unstable.
A critical parameter for controlling free layer stability is the magnetostriction of the free layer. Magnetoresistive sensors have an inherent compressive stress acting in a direction parallel with ABS. This compressive stress is due, among other things, to stresses generated by lapping when defining the air bearing surface (ABS). A positive magnetostriction in a free layer will induce a magnetic anisotropy in a direction that is perpendicular to the ABS and perpendicular to the desired biasing direction. This of course hinders free layer stability, making proper biasing of the free layer magnetization very difficult.
Free layers are generally constructed as a combination of a layer of CoFe and a layer of NiFe with the CoFe layer being located closest to the spacer layer (or barrier in the case of a tunnel valve). CoFe is known to have a positive magnetostriction, but is needed close to the spacer/barrier layer in order for the sensor to operate optimally. The NiFe layer has a negative magnetostriction which can be used to offset the positive magnetostriction of the CoFe layer somewhat. However, the overall thickness of the free layer is limited, because as the free layer becomes thicker its coercivity increases, thereby decreasing sensor performance. On the other hand, a certain minimum thickness of the CoFe layer must be maintained in order for the sensor to function properly. Therefore, it can be seen, that one can not simply increase the thickness of the NiFe layer of the free layer to create a desired negative or neutral magnetostriction.
Therefore, there is a need for a method for controlling the magnetostriction of a free layer without the need to increase thickness of the NiFe layer of the free layer. Such a method must not interfere with other magnetic properties of the free layer such that sensor performance would suffer. For example, such a method for adjusting magnetostriction should not result in an increased coercivity of the free layer, as such increased coercivity would decrease the sensitivity of the sensor.